Microsoft Corp. has made an unsolicited offer to buy Yahoo! Inc. for $44.6 billion, according to media reports. The move is believed to be Microsoft’s way to challenge Google Inc. in the online search and advertising markets. In the Valley, Yahoo! has its search marketing division in Burbank. Sunnyvale-based Yahoo! said in a statement that it would “carefully and promptly” study Microsoft’s bid, the Los Angeles Times reported.